Porous Science: How Does a Developing Chick Breathe Inside Its Egg Shell?

Introduction
Have you ever wondered how an unborn chick breathes inside its shell? Every animal needs oxygen to live, so the chick must get air somehow! When an animal—including a human—inhales, oxygen enters its lungs and is then distributed to all the different parts of its body. The animal's metabolism converts the oxygen into energy. During this process, a waste gas called carbon dioxide is produced. To get rid of it, the carbon dioxide is carried back to the lungs, where it is collected and exhaled. So not only must the chick have a way to let oxygen in, it also must somehow let carbon dioxide out. How does it do this sealed inside an eggshell?

Background
When oxygen enters an animal's lungs, it is shuttled and distributed by the bloodstream. It is also the bloodstream that carries carbon dioxide back to the lungs to be breathed out. Animals that grow inside their mothers, like humans, get their oxygen from their directly mothers. The blood stream of the baby animal and the mother are connected via an umbilical cord, which allows the baby to collect oxygen that his or her mother breathes in as well as use the mother's lungs to expel the carbon dioxide.

How do animals, such as chickens, which develop inside an egg outside of their mothers' bodies and therefore do not have umbilical cords, take in oxygen and get rid of carbon dioxide? Bird and reptile eggs have a hard shell. Directly under the shell are two membranes. Between the membranes is a small air cell, also called an air sack, filled with oxygen. As the animal develops it uses the oxygen, which must be replenished, and it also has to release carbon dioxide. How does this happen? Well, if you examine a chicken egg carefully with a magnifying glass, you'll see that there are tiny little holes, called pores, in the shell. In this activity, we'll see how those work to let the developing chick breathe.

Materials
• Large pot or bowl
• Water
• Blue food color
• Liquid dishwasher detergent
• Teaspoon measurers
• Three eggs (for best results, do not use freshly laid eggs, rather, use older, commercial eggs)
• Tongs or large spoon
• Cup
• Plate or paper towel
• Optional: a sensitive scale, such as a digital kitchen scale or a triple-beam balance that can measure tenths of a gram

Preparation
• Pour one and one half cups of water in a large pot or bowl.
• Add one quarter teaspoon of liquid dish detergent and one quarter teaspoon of blue food color. Mix well.

Procedure
• Carefully put the three eggs in the pot with the water, dish detergent and blue food color.
• Make sure that the eggs are submerged in the liquid. If part of the egg is above the surface of the water, mix together liquid dish detergent and blue food color with more water in the same proportions as you did before. Add this to the pot until the eggs are submerged.
• Set a timer for one hour or make a note of the time.
• After the eggs have soaked in the liquid for at least one hour, carefully lift one of them out of the liquid using the tongs or large spoon. How does the egg look?
• Crack the raw egg into a cup, being careful not to damage or crush the shell much.
• Set the empty eggshell on a plate or paper towel.
• Carefully inspect the inside of the shell. What do you see?
• Crack open the other two eggs in the same way. Look all around the inside of their shells, too. What do you see? Do all of the insides of the shells look the same? Are there noticeable differences?
• Extra:Do fresh eggs and aged eggs behave similarly? Buy a dozen eggs whose expiration date is at least two weeks away. Try this activity with half of the eggs right away. Let the other six eggs age in the refrigerator for two weeks. Repeat the activity with the aged eggs. How does the data compare between the fresh and the aged eggs?
• Extra: If pores in the chicken egg's shell allow materials to cross back and forth between the inside of the egg and the outside environment, then the air inside the egg could be replaced by water, and water is heavier than air. Using a scale that can distinguish changes as small as 0.1 gram, such as a triple-beam balance or high-quality electronic kitchen scale, weigh some eggs, then have an adult help you hard-boil them and weigh the eggs again. Did the eggs change weight? If so, how did they change weight? What does this say about the ability of the chicken egg to allow water to cross its shell?

Observations and results
Did all of the eggs have at least a few small blue dots on the inside of their shells? Were the dots mostly clustered in one or a few areas on the inside of each shell?

Directly under the chicken egg's shell are two membranes. When the eggs are laid by the mother they are warmer than the air, and as they cool the material inside the egg shrinks a little bit. This shrinkage is what pulls the two membranes apart, leaving behind the small air sack that is filled with oxygen. As the developing chick grows it uses the oxygen from the air sack and replaces it with carbon dioxide. The tiny pores in the shell allow the carbon dioxide to escape and fresh air to get in. The chicken egg has more than 7,000 pores in its shell to allow this to happen! These pores also allow water to go through the shell, which is why the dye appears as small dots on the inside of the shell, often clustered in certain areas, and why an egg after being hard-boiled would weigh slightly more than when it was raw. Also, freshly laid eggs do not allow water to penetrate as well as older, commercial eggs do, so fewer blue spots will probably be visible on the inside of fresher eggs compared with older ones.

Cleanup
Dispose of the raw eggs by pouring them down the drain. (The eggs should not be eaten because they were soaked with dishwater detergent.) Thoroughly clean any surface the raw eggs touched because they can carry salmonella.